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I believe this to be true and am sure someone somewhere has measured it (changing it in the process;-)What I mean is that I suspect that if we know that someone understands a subject, our default behaviour is to believe without really trying to understand. This is a useful trait (otherwise we'd all be wasting our time trying to understand the same things) - provided that the source is trustworthy.An untrustworthy source of information is often known as a "confidence trickster". Etc.

One more thing, there is a minority of scientists who believe that building a quantum computer will turn out to be out-and-out impossible.

However, if those scientists are right, the implication of not being able to build such a machine is that quantum mechanics itself, as a description of nature, is wrong. Either way, the stakes could not be higher.

One possible failure mode is the theoretical power required could exceed the light fluxs of the visible universe, that would be a bummer. Maybe in true supercomputer style, a formerly computational problem is merely converted into an I/O problem, the interface to the classical world might be too slow/imprecise/analog/noisy/random to pull useful results out of it. Nothing wrong with quantum theory at all, just not possible to interface usefully with the greater classical world.

Or the more practical engineering/accounting failure mode where it would simply be cheaper / faster / more efficient to use mass produced classical processor, possibly for any problem.

All theories are really just models of the universe, some work better and tell us more about how the universe does work. Quantum mechanics does tell us many very important things about our universe. Most importantly, and confusingly for everyone first learning it, our universe is not deterministic. Unlike dice, where we could predict with absolute certainty what the outcome would be if we collected enough data about the throw, we cannot do that in the quantum world. There are no "hidden variables" that we c

Note that not everyone rejects hidden variables. Claiming that QM implies a non-deterministic universe because of the absence of hidden variables is in fact subtly wrong. The dominant interpretation of QM (which claims the nonexistence of hidden variables) *assumes* nondeterminism, it doesn't conclude it. You can find a complete quote on the Wikipedia page for Superdeterminism, but there was an assumption in the design of the EPR experiment that assumed non-determinism as a means of preserving the free wi

I find superdeterminisim to be even stranger than quantum mechanics, and too close to creationism/intelligent design to me. The answer to every question simply becomes " because that's the way God did it" Or "because that's the way the initial conditions set it up". It seems downright psuedo scientific and even more of a philosophical intrusion.
I also find the lack of free will to be more mind boggling, with more bad implications in real life than quantum mechanics.

I find non-determinism to be stranger. In a deterministic world, you can always ask what caused anything that you observe. In a non-deterministic world, you observe something, and apparently it happened without a cause, it could have just as well been one of several other outcomes. If you accept non-determinism, it's not free will that results, it's that your will is governed by random chance and not by cause and effect.

If our brains are quantum mechanical devices, that doesn't mean that all of our thoughts are random. Just that they are not 100% reliable. That seems to be my experience. Plus, you can force quantum mechanics non determinism to work for you to get some sweet results: like, uhmm, quantum computers. I'm okay my thoughts not being completely logical, and I'll admit sometimes my effects are not very closely related to any external causes, but that doesn't mean I have a random number generator in there.

Nope. They are not, if quantum mechanics is correct. Science is built upon science. If the lowest level of interaction of particles is deterministic then they all are. If it is not, then they are not(but for all practical purposes they certainly can be considered to be deterministic, but we know there is always a minisucle probability > 0 that something odd will happen.

Yes, but "there's a signal that propogates at infinite speed and yet can't be used for communication" is a road you really don't want to go down.

Bell's theorem forces you to choose between rejecting locality and rejecting counterfactual definiteness (i.e. the idea that there is a pre-existing property that is "waiting" for our measuring to find out what it "really was" the whole time).

If anyone were able to convince HR to put such a requirement on a job position, it would surely be the most awesome fail possible, and failblog would probably make a t-shirt out of it... In fact, making a t-shirt out of it whether true or false would be absolutely awesome. Somebody, go make a t-shirt out of it, I want to wear one to work and make everyone scratch their heads.

....[Phone recording] You reached the secure communications quantum cryptography help desk. We detected that you pressed "Reset my password". This option requires a transfer to teh 8th dimension. We are now connecting you to Lord John Whorfin...

[Lord John Whorfin] May I pass along my congratulations for your great interdimensional breakthrough. I am sure, in the miserable annals of the Earth, you will be duly enshrined. How may I help you?

Everyone always talks about the differences between a standard computer and a Quantum computer. Graphics cards are good for floating point numbers, why can't we have a Quantum Card to handle quantum operations? Does it really have to be one or the other?

You cannot get the power of a large quantum computer by repeatedly running a small quantum computer, because you cannot store intermediate quantum results in classical memory. However, real applications will probably always be combinations of classical and quantum computers, because for a lot of problems quantum computers don't have an advantage over classical computers, so it would be wasteful to do those parts a classical computer can do well on a quantum computer instead. As an example, when running Shor

Personally, I'll be happy with plain old computing on nano-structures or a Photon-Computer. Nothin' special, just wicked fast, all Unixy on the insides, and small enough to fit in my jeans change pocket.http://en.wikipedia.org/wiki/Photon_computer [wikipedia.org]

Sorry to be so negative but in my opinion the article is horrible. It doesn't explain anything unless you think bad analogies and jovial metaphors help you understand things better. After having read it, I don't know a single qubit more about quantum computers than before.

Actually, I imagine that quite a few CEOs did take some hard science or engineering courses, at least in tech companies. Maybe I just hope that is the case. I would not expect bank or insurance company CEO's to even bother with reading an article about quantum computing, as simplified as it might be.

Sorry to be so negative but in my opinion the article is horrible. It doesn't explain anything unless you think bad analogies and jovial metaphors help you understand things better. After having read it, I don't know a single qubit more about quantum computers than before.

Yeah, with a 350-pixel-wide web page (yes, the entire page), and an opening like this, I can't imagine why nobody would read any further:

Time machines - oh, boy!
Steady on Sam, I love science fiction as much as the next geek but I'm not talking about Quantum Leap here. This is even more exciting than time travel.
OK, so what is this quantum computing lark then?
Quantum computing and quantum information processing are research efforts that seek to exploit quantum mechanical phenomena to perform tasks such as massively parallel computing. The quantum research field also encompasses quantum cryptography, which utilises quantum phenomena to guarantee secure communications.

What are these quantum phenomena you talk of?
Tsk! Clearly weren't paying attention in physics class were you? [...]

Tip to new writers: you aren't witty, you aren't funny, you aren't entertaining. Leave your antics out of the writing and cover the subject matter so well that its inherent nature will be interesting to the reader.

I agree as well. I've read better articles on quantum computing.
It seems no one can really explain how calculations are actually performed. They talk about qubits and how they have 3 states, but no one ever goes into how the 3rd superposition is actually of any use. It's frustrating at times. I could rant on, but I wont.

So here's the way I understand it - or rather I probably misunderstand it:

.

Quantum Computing - because it stores superpositions of bits, which can represent all values, can work work on data with not just a single value, but with all possible values, thus doing stuff effectively in parallel

Before you stop reading - it gets better...

This is good for solving "what-if" problems - (which I think is technically described as "NP-Hard" - but I'm not positive) - or problems which can only be solved be tryin

That all sounds fine and good, but if you can't observe a quantum system without altering it how the hell do you read the answer? I don't understand how you can entangle two photons and get anything useful out of them because you can't observe one without altering the other. How do you read out anything useful in a quantum system?

Let me take a run at explaining quantum computing less awkwardly than the article.

A quantum bit (qbit) may be in a 0 state, a 1 state or any linear superposition (combination) of the two, eg. 0 + i1. When measured, the outcome of the measurement can only be 0 or 1 with the probabilities of each being governed by the ratio of contributions to the qbit from the 0 and 1 components.

One qbit can usefully encode one bit of classical information (this is the point that most articles on the subject muddle up). Enta

As obvious as it may be to include a "picture" of an atom -- a Rutherford model [wikipedia.org] -- it seems terribly incorrect to use it as the primary image to be associated with a quantum-mechanical phenomenon. Though I guess it's good enough to make the article feel "science-y".

I can't help but recall Wyoh Knott, the heroine of Heinlein's "The Moon is a Harsh Mistress", who conceived of an electron as "about the size and shape of a small pea".

It's also a picture of an atom that doesn't exist. Never mind that the electrons are enormous and have circular orbits. There are 2 of one kind of nucleon and 3 of the other kind, with 4 electrons that all seem to be in the same shell.

So, the two possible atoms are Lithium-5 (-1) or Helium-5 (-2). Both Lithium-5 and Helium-5 are highly unstable. Both of them should have two electrons in one shell and two in higher-energy shell. The -2 state of helium would be challenging to produce, to say the least.

I tried to RTFA, but the author wants to be too cute and buddy-buddy with me. When you're trying to learn something new, little jabs and crappy jokes prevent me from getting into the learning zone. Stick with the wikipedia article.

Have to agree with the comments above, that article is pretty useless.

Coincidentally, though, at a university book sale a few weeks ago, I picked up a copy of N. David Mermin's Quantum Computer Science: An Introduction, for just $5 (seems to be about $30 on Amazon) and I can't recommend it highly enough. It's an intro to quantum computing textbook, about 200 pages, written specifically for people who have CS or math (as opposed to physics) backgrounds, and while it's almost impossible to get into the nitty-gritty of why quantum computing works without a lot of quantum mechanics esoterica, this book does a great job of explaining how it works (which is plenty complicated on it's own).

It's not a light read (it's a textbook, after all), and contains some serious math, but it's nothing someone with a college education can't handle and it really helped me understand this whole mess better than any popular news article.

Incidentally, from Mermin's website [cornell.edu], you can download his lecture notes [cornell.edu] at no cost. The book is directly based on the lecture notes and, as far as I recall, the notes are pretty good. I took the class while he was working on the book, so all we had to work with was the lecture notes (which have since undergone some revisions), which were essentially a beta version of the book's text.

It should be reasonably understandable to someone with a good CS and mathematical background but limited physics background. (Likewise, it should be reasonably understandable to someone with a good physics background but relatively little CS.) The course was designed to be taken by both CS and physics students. I think it was fairly challenging for the Cornell CS undergrads that were in the course, but your mileage may vary.

...my brain processes became quantumly entangled.
Since quantum computing itself is partially inexplicable, and building a physical machine is currently impossible, we probably won't be seeing this in the near future unless it is on an episode of Star Trek OR if they will use it to make Wall Street trading machines faster.

Unfortunately, the author does not seem to have understood the concept of entanglement. Correlations between particles (even perfect ones) are also found in classical physics as the example with the socks implies. Quantum entanglement, however, is much more subtle, and distinguishing between useful entanglement and useless classical correlations is typically a highly nontrivial tasks.

As someone with a couple physics degree, I find this stuff both exciting and confusing. Exciting because delving into such fundamental physics is creating some beautiful results. Confused because either something is wrong with my understanding of quantum mechanics (unlikely), or we are being snowed with pie-in-the-sky promises, in order to secure funding.

Entanglement for secure channels of communication I believe. Quantum "computing" in the sense we usually think of computing looks phony.

So Ive always read that quantum computers excel in parallel calculations like code-breaking and such, but if its probability calculations that it are it's bread and butter, would that extend to weather prediction modeling? I'd love for the Weather service to get as good as they were in Back to the Future II, weather predictions accurate to the second.

Won't happen. While better computers help to some degree and building better models helps even more, weather is fundamentally a chaotic system. There is a hard upper limit based on the quantity and accuracy of the data. An exponential increase in overall quality of data yields a linear increase in the quality of the prediction.

To make up some numbers to illustrate the point...So if we have 10^4 weather stations to have 48 hours of good accuracy, it might take 10^5 weather stations to achieve 60 hours o

Even with the best and fastest computer you'll not get weather prediction accurate to the second. That's because weather is a chaotic system, and you'd need an insane amount of measurement data to be that accurate (and probably would have to predict human behaviour as well!)

This shared state means that a change applied to one entangled object is instantly reflected by its correlated fellows - hence the massive parallel potential of a quantum computer.

Unless I missed some major recent development, modifying an entangled particle and "instantly" observing the effect on its correlated partner is precisely what you cannot do with an entangled pair. Gets into that whole pesky faster than light communication thing that makes causality not work.

No, he's just wrong, or at the very least severely dumbing down the real picture for the sake of placating the lay audience (which Slashdot is, generally speaking, not). There's a few examples of this already on the first page - I didn't even make it to the second...

That seems to be a problem with a lot of modern science: correct, brief, understandable to the layman. Pick two.

There is a theory, probably via Roger Penrose (or not, or both), that biological brains are so curiously different from standard computers and good at diffuse problems like pattern matching exactly because they are tapping into quantum entanglement as a material for decision making. So... build ye a quantum computer and see it stare back from the (quantum) abyss at you... (or not)

"This shared state means that a change applied to one entangled object is instantly reflected by its correlated fellows"

Why, oh why, is this nonsense repeated again and again. If you change one entangled particle, you do not change the other. For example, if you have two spins entangled in a way so that if one is measured "up" the other is measured "down" and vice versa, and you turn the one spin around (without measuring it) then you'll have an entangled state where if you measure the firs

Thanks for the post. I have another silly question though - which I'm sure others are asking themselves. If you have particle A and particle B, entangled in spin say, then when you measure A the entangled state collapses, you get the spin for A, and you know what the spin for B will measure. OK so far?

Silly question is, if we haven't measured anything yet, how do we know there is some 'spooky action at a distance'? Maybe particle A's spin was ALWAYS up, and B's was ALWAYS down, you just didn't know it

The article contained a little too much "future tense" for my liking. ISTR people have been talking about quantum computing, entanglement and qubits for, what?, a decade or more. Now I'm a patient man but it still seems to be couched in phrases like "we will..." "could..." "would..." "... is being researched" "... is theoretically..." . I recall back in the 60's when lasers were the wonder of the time, they got a bit of a reputation for being a solution in search of a problem - though that's obviously c

So does that mean that some time in the next 50 years, we'll have quantum computers crunching massively paprallel problems, such as decrypting all our previously secure communications, manipulating all the pixels of a video feed in real-time, right off the sensor or even with an entanglement USB peripheral that takes the place of all our networking and communications systems - providing instantaneous point-to-point links between pairs of chips?

I think you're expecting way too much from quantum computing:

In 50 years we'll probably be able to decrypt all our (current) secure communications anyway, with or without quantum computers. Of course, by then we'll be using larger keys (or probably better algorithms that provide better security with smaller keys). And if quantum computers start to become feasible, we can start moving to encryption systems that cannot be broken by quantum computers (the McEliece cryptosystem is one candidate).

You could well be right. However, from the sense of optimism in the article these are the sorts of things I would come to expect. If on the one hand they're making statements using words like "revolutionise" and "massively parallel" thn they've got to back 'em up!

Personally I'd say that it's still too early to say if QC will bear fruit - just we haven't yet seen any major benefits of DNA mapping (people still get cancer). It would just be nice to read a piece that wasn't trying for the hard sell, and was

...and I read most of the fine article. It puts too much emphasis on quantum entanglement, which is useful for quantum cryptography, but not as important as quantum interference. It's the weird quantum states of the individual qubits that interfere with each other, that make a quantum computer. If you can figure out how to encode information into the quantum state of a qubit, and get a bunch of them to interact in a given way, you get the quantum interference to cancel out the information you don't want,

As others have noted, the author's explanation of entanglement is faulty. IMO the key fault (although there are others) is the implication that entanglement can be used for (perhaps super-luminal) communication.

Perhaps only a minority of scientists think building large scale quantum computers is impossible, but I think a majority of physicists think it is impossible, I certainly do. I strongly disagree with the idea that the only way building large scale quantum computers would be impossible i

The review paper I was thinking of was by Maximilian Schlosshauer. It was called "Decoherence, the measurement problem, and interpretations of quantum
mechanics". He has also written a book about quantum decoherence.

Large scale quantum computers could easily factor large numbers thus rendering perhaps all of our current encryption systems obsolete. I have no idea what they would be replaced with but it seems at least possible that anyone who wanted to communicate securely would need to use a quantum computer.

I don't think that it's likely that quantum computers (if built) will be necessary for secure communication. There are cryptosystems (check out McEliece [wikipedia.org]) that can be run relatively fast in classical computers and are based on the difficulty of solving NP-hard problems. Quantum computers would not offer exponential speedups for these kinds of problems (that's not actually proved, but it's even more certain than P!=NP, I think).

a) Its more than a small minority of scientists who do not believe in quantum computation, even if being a minority would make a difference in science. Making it sound like these people are a kind of weirdos does not give enough respect to a lot of great minds. There are practical reasons we will collide with and mother nature may hold more more us than we expect. It seems that Quantum mechanics holds for small systems and for massively uniform systems. I, as many others expe

The set of problems you can in principle solve with a quantum computer is exactly the same as you can solve with classical computers. The best proof of this is that you can simulate a quantum computer with a classical computer (and vice versa). However, as far as we know you cannot simulate a quantum computer on a classical computer in polynomial time.

Quantum computers only offer better speeds; a quantum computer can always be simulated by a classical computer. However, storage and run time of the simulation grows exponentially with the size of the quantum computer being simulated, so this is not feasible in practice.

The reverse is also true. A quantum computer (when/if built) will be able to run any classical algorithm, since it's possible to implement a classical NAND gate using quantum gates. It'd be a huge waste, however, to use quantum gates this wa